Microsoft has launched a pilot program in which some visitors to the main Windows download page are being asked to let the software maker check to see whether their copy of the operating system is licensed.

Visitors do not have to participate in the program to get their downloads. They'll also get their downloads if they do participate and their copy of Windows turns out to be unlicensed but Microsoft said the program is a first step in trying to provide a better experience for customers using legitimate copies of Windows.

Since the program is optional at this stage, Microsoft expects that most of those who know their software is bogus will not take part.

David Lazar, a director in the Windows client unit, said: "I would expect that people who know they are running pirated Windows are going to be very interested to know what we are doing, but they could easily choose to not opt in."

Those whose copies are found not to be genuine will be encouraged to go back to the company from whom they bought the PC or software upgrade. They'll also be given other information on obtaining genuine software before being allowed to download whatever software they were seeking. In its current form, the program offers no particular benefit for those who are running licensed software.

Eventually, Microsoft could make the program mandatory. The company could also prevent those that have unlicensed copies of Windows from downloading software updates on Microsoft's site.

Still, Lazar said, it is a sensitive group of customers Microsoft is targeting with the program - namely, people who bought a computer that they thought had a legitimate copy of Windows but are somewhat unsure. Microsoft wants those people as customers, so it wants to be sure to treat them kindly, even as the company seeks to encourage legitimate Windows use.

"We've got to pilot the infrastructure," Lazar said. "We're kind of making a value judgment [on whether a copy of Windows is genuine]. We want to make sure we get that correct and the experience for everyone is a positive one."

Lazar said Microsoft's goal is to have 20,000 users go through the procedure before deciding how to expand or modify the program, an effort that he estimates could take anywhere from six weeks to three months.

Dubbed the Windows Genuine Advantage, the program is part of Microsoft's overall antipiracy campaign, Lazar said, noting that the company's efforts fall into three categories: education, enforcement and engineering. The new program is part of the company's effort to engineer its software to encourage legitimate use, Lazar said.

The Business Software Alliance, of which Microsoft is a key member, estimates that piracy costs software makers $29bn annually, although some have taken issue with the means by which the group calculates those figures.

Lazar stressed that Microsoft is not blocking access to any updates, in particular to Windows XP Service Pack 2, which the company is encouraging all customers to upgrade to. The company is not requiring customers to have a genuine copy of Windows to get SP2, though it has blocked a few registration codes that have been known for several years as pirated.